Synthesis Of Single-walled Carbon Nanotubes Research Articles

Molecular Dynamics of Catalyst-Free Edge Elongation of Boron Nitride Nanotubes Coaxially Grown on Single-Walled Carbon Nanotubes.

Recent advances in low-dimensional materials have enabled the synthesis of single-walled carbon nanotubes encapsulated in hexagonal boron nitride (BN) nanotubes (SWCNT@BNNT), creating one-dimensional van der Waals (vdW) heterostructures. However, controlling the quality and crystallinity of BNNT on the surface of SWCNTs using chemical vapor deposition (CVD) remains a challenge. To better understand the growth mechanism of the BNNT in SWCNT@BNNT, we conducted molecular dynamics (MD) simulations using empirical potentials. The simulation results suggest that spontaneous BN nucleation is unlikely to occur on the outer surface of the SWCNT when we assume only vdW interaction between the BN and SWCNT layers. However, we observe the elongation of the BNNT when a short BNNT is provided as a seed nucleus on the SWCNT. This grown BNNT structure, with its sharply cut edges, aligns with experimental observations made using transmission electron microscopy (TEM). Moreover, the edge-reconstruction process favors zigzag B edges, which exhibit low edge energy according to the ReaxFF potential. Our simulation successfully provides insights into the catalyst-free growth process of this one-dimensional vdW heterostructure.

Double-Layer Support Strategy for Enhancing the Lifetime of Fe Catalyst Nanoparticles in Synthesis of Single-Walled Carbon Nanotubes

Double-Layer Support Strategy for Enhancing the Lifetime of Fe Catalyst Nanoparticles in Synthesis of Single-Walled Carbon Nanotubes

An efficient approach toward production of near-zigzag single-chirality carbon nanotubes.

Synthesizing single-walled carbon nanotubes (SWCNTs) with a narrow chirality distribution is essential for obtaining pure chirality materials through postgrowth sorting techniques. Using carbon monoxide chemical vapor deposition, we devise a ruthenium (Ru) catalyst supported by silica for the bulk production of SWCNTs containing only a few (n, m) species. The result is attributed to the limited carbon dissociation on the supported Ru clusters, favoring the growth of only small-diameter SWCNTs at comparable growth rates. The resulting materials expedite high-purity single chirality separation using gel chromatography, leading to unprecedented yields of 3.5% for (9, 1) and 5.2% for (9, 2) nanotubes, which surpass those separated from HiPco SWCNTs by two orders of magnitude. This work sheds light on the large-quantity synthesis of SWCNTs with enriched species beyond near-armchair ones for their high-yield separation.

Towards the preparation of single-walled carbon nanotubes with average diameter of 1.2 nm

Preparation of single-walled carbon nanotubes (SWNTs) with diameters ranging from 1.2 to 1.5 nm has been highly demanded for field-effect transistors with superior properties. In the work, we report the chemical vapor deposition synthesis of SWNTs with an average diameter of 1.2 nm from a rationally designed nickel catalyst supported by magnesia. The high metal dispersion, the suitable reaction temperature, and the use of a methane carbon source co-determine the SWNT diameter distribution. Furthermore, through the synergistic application of poly(9,9-dioctylfluorene-2,7-diylalt-pyridine-2,6-diyl) wrapping and ultracentrifugation, SWNTs with enriched (10,8) species of 1.24 nm are successfully extracted, which is attributed to the selective interaction between the polymer molecules and the targeted SWNTs. This work not only sheds light on the growth of SWNTs with relatively large diameter, but also paves the way towards the preparation of high purity (n,m) species that could meet the requirements of advanced high-tech applications.

Evolution of catalyst design for controlled synthesis of chiral single-walled carbon nanotubes.

Single-walled carbon nanotubes (SWCNTs) possess superb properties originating from their unique chiral structures. However, accurately controlling the structure of SWCNTs remains challenging due to the structural similarities of their chiral structures, which hinders their widespread application in various fields, particularly in electronics. In recent years, much effort has been devoted to preparing single chiral SWCNTs by adopting three constructive strategies, including growth condition control for structurally unstable liquid catalysts, employing stable solid catalyst design, and pre-synthesis of carbon seeds with a well-defined shape. This review comprehensively discusses the state-of-the-art developments in these approaches as well as their advantages and disadvantages. Moreover, insights into the key challenges and future directions are provided for acquiring chirally pure SWCNTs.

Growth of Single-Walled Carbon Nanotubes from Solid Supported Heterogeneous Catalysts: Achievements and Challenges

Single-walled carbon nanotubes (SWNTs) have gained worldwide attention because of their exceptional electronic, optical, and mechanical properties, which are contingent on their structures denoted by chiral indices (n, m). To fully harness the extraordinary properties and advance SWNT-based technology, large scale synthesis of SWNTs with a narrow chirality distribution or even a specific (n, m) is highly demanded. Among various SWNT preparation techniques, chemical vapor deposition growth from solid supported heterogeneous catalysts has emerged as a promising approach in terms of large yield and (n, m) structure control. In this review, we provide an overview of the progress in developing heterogeneous catalysts for bulk synthesis of SWNTs. Firstly, the advantages and disadvantages of using solid supported catalysts for chemical vapor deposition growth of SWNTs are discussed. Secondly, the key components in supported catalysts, including the metallic component and the nature of support material, as well as their respective roles in affecting the catalyst performances and SWNT chirality distribution, are highlighted. Finally, the current challenges and future directions in the field of SWNT synthesis from solid supported catalysts are proposed.

Boosting CO-based synthesis of single-walled carbon nanotubes with hydrogen

Aerosol (floating catalyst) chemical vapor deposition method based on CO disproportionation (the Boudouard reaction) is one of the most promising techniques for the synthesis of single-walled carbon nanotubes (SWCNTs), especially in the form of thin films. However, despite its advantages (i.e., high quality of nanotubes, absence of double- or multi-walled nanotubes, and high controllability of the process), synthesis based on CO disproportionation fails to provide high-yield production. In this work, we examined the effect of hydrogen, admixed to the CO atmosphere as a growth promoter, on the synthesis and properties of SWCNTs. Using optical spectroscopy techniques, conductivity tests, TEM and SEM analysis, and thermodynamic calculations, we revealed hydrogen affects both catalyst activation and nanotube growth. We found a positive H2 effect in two different temperature regimes: the yield increased by a factor of ∼15 at a low temperature (880 °C), which, after doping, led to one of the lowest ratios of the equivalent sheet resistance (R90) and yield (i) and SWCNT lengthening by a factor of 2.4 leading to a 3-fold decrease in an R90 coupled with a noticeable increase in the yield at a high temperature (1000 °C) (ii). We believe the presented results are fruitful for the fundamental understanding of the mechanism for nanotube growth and practical opportunity to fine-tune the production rate of SWCNT films.

Upcycling plastic polymers into single-walled carbon nanotubes from a magnesia supported iron catalyst

Plastic waste has posed an ever-growing environmental threat to the United Nations’ sustainable development goals. Converting plastic polymers into high value chemicals like carbon nanotubes is a promising approach for recycling without carbon dioxide emission. Using a two-stage pyrolysis-catalytic reactor, we herein report the transformation of five types of plastic polymers into high-quality single-walled carbon nanotubes (SWNTs) with the assistance of a rationally designed heterogeneous catalyst, i.e., an iron metal catalyst supported by porous magnesia. Because of the high metal dispersion and the large carbon solubility of the active component, the heterogeneous catalyst affords the formation of uniform metal particles which can sustain high carbon fluxes, resulting in the synthesis of high quality SWNTs. More importantly, the approach is also demonstrated with mixtures of plastic polymers. This work helps gain deep insights into the mechanisms of plastic polymer conversion to SWNTs, accelerating the progress toward a sustainable plastic economy.

Supported catalysts derived from cobalt phyllosilicates for chemical vapor deposition growth of single-walled carbon nanotubes

Chemical vapor deposition (CVD) growth of single-walled carbon nanotubes (SWNTs) using supported catalyst is problematic due to possible metal coalescence during reaction. Thus, the development of supported catalyst that resists sintering is crucial for synthesizing SWNTs with a narrowly distributed chirality. Here, silica-supported phyllosilicate materials containing Co are prepared for catalytically growing carbon nanotubes. Owing to the incorporation of Co in the phyllosilicate, only part of Co2+ is reduced under CVD reaction environment, leading to the formation of immobilized metallic particles which subsequently catalyze the growth of SWNTs at 850 °C. With the addition of Pd promoter, the reducibility and carburization of the Co particles is enhanced, leading to the preferential synthesis of (6, 5) SWNTs at 600 °C. This work extends the application of phyllosilicates in catalytic synthesis of SWNTs, thus providing a new avenue for chirality-selective synthesis of SWNTs.

Nitrogen-Doped Single-Walled Carbon Nanotubes by Floating-Catalyst CVD Process

Due to the properties of single-walled carbon nanotube (SWCNT), thin film SWCNTs have been used for many versatile applications. Direct synthesis of the nanotube films with desired electronic structures without prerequisite of post treatment process then becomes one important step. Here, we present the synthesis of nitrogen-doped SWCNTs by floating-catalyst chemical vapor deposition process. With low nitrogen level incorporated into the sp2 carbon network, substitutional and pyridinic nitrogens become predominance. The electronic structure of N-doped SWCNT film could be changed into n-type doping. The localized structures of carbon and nitrogen bonding environments were also revealed by x-ray absorption spectroscopy. The formation of p-n junction was also obtained from the I-V characteristic of N-doped SWCNT heterojunction diode revealing n-type behavior of the sample.

Separation of left- and right-handed semiconducting single-walled carbon nanotube enantiomers using achiral amino acid surfactant

Synthesized single-walled carbon nanotubes (SWNTs) are racemic mixtures of various (n, m) species with different electronic types, which greatly impeded their practical applications and fundamental research. In this paper, achiral amino acid surfactant sodium N-cocoyl sarcosinate (N-CS) not only displayed selectivity toward s-SWNTs but also could further separate the sorted s-SWNTs by the chirality indices (n, m) and left- and right-handedness with the aid of density gradient ultracentrifugation (DGU). The specific molecular structure of N-CS offered the possibility of recognizing the left- and right-handedness of SWNTs. The enantiomer separation mechanism was illustrated by Molecular dynamics simulations.

Modelling of the Flame Synthesis of Single-walled Carbon Nanotubes in Non-premixed Flames with Aerosol Catalyst

The use of aerosol catalyst in the flame synthesis of carbon nanotube (CNT) is known to yield single-walled CNT (SWCNT) that is useful for various applications. Modelling works are needed to optimize operating conditions for SWCNT growth but are unavailable. Therefore, a baseline model for the aerosol-catalyst system in flames is developed and the effect of oxygen on SWCNT growth is investigated. A baseline flame model for a normal diffusion flame with 24% oxygen concentration at the inlet is established via Computational Fluid Dynamic simulation. A dispersed phase model (DPM) is employed to simulate the entrainment of catalyst particles. The flame model is coupled with a published CNT growth rate model to predict the CNT growth rate at each particle. Inlet oxygen concentration is varied from 19% to 27% to study the effect of oxygen on SWCNT growth. Satisfactory validation of the baseline flame shape and temperature is established. Results show that particle 3 for the baseline case yields the highest CNT length compared to other particles due to the suitable path for the synthesis. The particles are classified based on the shortest time residence, moderate and longest time residence. Increasing oxygen concentration from 19% to 27% results in a 30% decrease in CNT length for particle 3 for each inlet condition due to lower carbon precursor and composition in the flame. Furthermore, the results showed that regardless of burner operating conditions, high SWCNT growth is consistently predicted between 120-140 mm HAB, which indicates the existence of an optimum range of species concentration for SWCNT growth in aerosol-based flame synthesis. Thus, it can be inferred that SWCNT growth in the aerosol–based method is highly dependent on carbon source and moderately dependent on temperature

Role of Hydrogen in Ethylene-Based Synthesis of Single-Walled Carbon Nanotubes.

We examined the effect of hydrogen on the growth of single-walled carbon nanotubes in the aerosol (a specific case of the floating catalyst) chemical vapor deposition process using ethylene as a carbon source and ferrocene as a precursor for a Fe-based catalyst. With a comprehensive set of physical methods (UV-vis-NIR and Raman spectroscopies, transmission electron microscopy, scanning electron microscopy, differential mobility analysis, and four-probe sheet resistance measurements), we showed hydrogen to inhibit ethylene pyrolysis extending the window of synthesis parameters. Moreover, the detailed study at different temperatures allowed us to distinguish three different regimes for the hydrogen effect: pyrolysis suppression at low concentrations (I) followed by surface cleaning/activation promotion (II), and surface blockage/nanotube etching (III) at the highest concentrations. We believe that such a detailed study will help to reveal the complex role of hydrogen and contribute toward the synthesis of single-walled carbon nanotubes with detailed characteristics.

Synthesis of single-walled carbon nanotubes functionalized with platinum nanoparticles to sense breast cancer cells in 4T1 model to X-ray radiation.

In recent years, various types of radiosensitizers have been developed to address the challenges of cancer radiotherapy. Here, platinum-functionalized oxygenated single-walled carbon nanotubes (O-SWCNTs-Pt) coated with folic acid (FA) and bovine serum albumin (BSA) (O-SWCNTs-Pt-BSA-FA) were synthesized, characterized, and used as radiosensitizers to improve the therapeutic efficacy of X-rays in a mouse model of breast cancer (4T1) in vitro. The nanosensitizer was characterized by different techniques, such as transmission electron microscopy (TEM), selected area electron diffraction (SAED), dynamic light scattering (DLS), zeta potential, X-ray diffraction (XRD), ultraviolet-visible (UV-visible), and Fourier transform infrared (FTIR) spectrometry. The evaluation of cell viability with nanocarriers O-SWCNTs-BSA, O-SWCNTs-Pt-BSA, Pt-BSA-FA, and O-SWCNTs-Pt-BSA-FA is reported at the concentrations of 10, 30, and 90 μg/mL by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay in the presence and absence of X-rays at 4 and 8 Gy. The results showed that administration of O-SWCNTs-BSA, O-SWCNTs-Pt-BSA, Pt-BSA-FA, and O-SWCNTs-Pt-BSA-FA + 8 Gy at a concentration of 90 μg/mL reduced survival by 75.31, 65.32, 67.35, and 60.35%, respectively. O-SWCNTs-Pt-BSA-FA has a hydrodynamic size of 88.57 nm and a surface charge of -29 mV, which indicates special stability. Compared with O-SWCNTs-BSA, O-SWCNTs-Pt-BSA, and Pt-BSA-FA, it has very strong cell-killing activity in the 4T1 cell line. It is also noteworthy that SWCNTs can act as a controlled release and delivery system for PtNPs due to their unique properties and easy penetration into biological membranes. As a result, thenew nanosensitizer may play a role in cancer treatment in conjunction with radiotherapy technology. Graphical abstract.

(n, m) Distribution of Single-Walled Carbon Nanotubes Grown from a Non-Magnetic Palladium Catalyst

Non-magnetic metal nanoparticles have been previously applied for the growth of single-walled carbon nanotubes (SWNTs). However, the activation mechanisms of non-magnetic metal catalysts and chirality distribution of synthesized SWNTs remain unclear. In this work, the activation mechanisms of non-magnetic metal palladium (Pd) particles supported by the magnesia carrier and thermodynamic stabilities of nucleated SWNTs with different (n, m) are evaluated by theoretical simulations. The electronic metal-support interaction between Pd and magnesia upshifts the d-band center of Pd, which promotes the chemisorption and dissociation of carbon precursor molecules on the Pd surface, making the activation of magnesia-supported non-magnetic Pd catalysts for SWNT growth possible. To verify the theoretical results, a porous magnesia supported Pd catalyst is developed for the bulk synthesis of SWNTs by chemical vapor deposition. The chirality distribution of Pd-grown SWNTs is understood by operating both Pd-SWNT interfacial formation energy and SWNT growth kinetics. This work not only helps to gain new insights into the activation of catalysts for growing SWNTs, but also extends the use of non-magnetic metal catalysts for bulk synthesis of SWNTs.

Selective synthesis of large diameter single-walled carbon nanotubes on rice husk-derived catalysts

Selective synthesis of large diameter single-walled carbon nanotubes (SWCNTs) is desired for a variety of applications, but most catalysts can only synthesize SWCNTs with a diameter less than 2 nm. Here, a low cost and green catalyst (Co/RH-SiO2) was prepared by using rice husk-derived SiO2 as the catalyst support, which selectively synthesized large SWCNTs (dt > 2 nm) with a percentage of 70.7 % among all tubes grown at the high growth temperature of 950 ℃. The carbon yield produced on this catalyst containing 1 wt% Co was 4.49 wt%. In contrast, commercial SiO2 supported Co catalyst (Co/Cab-SiO2) synthesized smaller SWCNTs with a broad diameter distribution under the same growth condition. The detailed characterizations of catalysts showed that Co/RH-SiO2 catalyst is unique with the abundant small mesopores (2–4 nm) and reducible Co hydrosilicate and surface Co silicates highly dispersed in the pore structures. The reduction of Co hydrosilicate and surface Co silicates facilitates the formation of large Co clusters, which are stabilized by irreducible Co silicates and small mesopores, leading to the growth of large-diameter SWCNTs. Results in this study provide useful insights to develop low cost and green catalysts from biomass waste for the selective growth of SWCNTs.

Polyethyleneimine-Starch Functionalization of Single-Walled Carbon Nanotubes for Carbon Dioxide Sensing at Room Temperature.

There is an ever-growing interest in the detection of carbon dioxide (CO2) due to health risks associated with CO2 emissions. Hence, there is a need for low-power and low-cost CO2 sensors for efficient monitoring and sensing of CO2 analyte molecules in the environment. This study reports on the synthesis of single-walled carbon nanotubes (SWCNTs) that are functionalized using polyethyleneimine and starch (PEI-starch) in order to fabricate a PEI-starch functionalized SWCNT sensor for reversible CO2 detection under ambient room conditions (T = 25 °C; RH = 53%). Field-emission scanning electron microscopy, high-resolution transmission electron microscopy, Raman spectroscopy, and Fourier transform infrared spectroscopy are used to analyze the physiochemical properties of the as-synthesized gas sensor. Due to the large specific surface area of SWCNTs and the efficient CO2 capturing capabilities of the amine-rich PEI layer, the sensor possesses a high CO2 adsorption capacity. When exposed to varying CO2 concentrations between 50 and 500 ppm, the sensor response exhibits a linear relationship with an increase in analyte concentration, allowing it to operate reliably throughout a broad range of CO2 concentrations. The sensing mechanism of the PEI-starch-functionalized SWCNT sensor is based on the reversible acid-base equilibrium chemical reactions between amino groups of PEI and adsorbed CO2 molecules, which produce carbamates and bicarbonates. Due to the presence of hygroscopic starch that attracts more water molecules to the surface of SWCNTs, the adsorption capacity of CO2 gas molecules is enhanced. After multiple cycles of analyte exposure, the sensor recovers to its initial resistance level via a UV-assisted recovery approach. In addition, the sensor exhibits great stability and reliability in multiple analyte gas exposures as well as excellent selectivity to carbon dioxide over other interfering gases such as carbon monoxide, oxygen, and ammonia, thereby showing the potential to monitor CO2 levels in various infrastructure.

Combination effect of growth enhancers and carbon sources on synthesis of single-walled carbon nanotubes from solid carbon growth seeds

In the synthesis of highly crystalline single-walled carbon nanotubes (SWCNTs), high growth temperatures are preferred, while the formation of impurity amorphous carbon (a-C) causes the termination of SWCNT growth and degrades its properties. To remove this by-product, H2O and CO2 have been employed in metal-catalyzed SWCNT growth systems because of their oxidizing ability. Recently, nonmetallic nanoparticles have become one of the growth seed candidates because of their high melting points and fewer metal impurities. In this study, by using nanodiamond-derived carbon nanoparticles as the solid growth seeds, we investigated the effect of CO2 and H2O on high-temperature SWCNT growth with two types of carbon-source supplies: C2H2 and C2H4. In this growth system, H2O showed oxidizing ability to etch a-C with either carbon sources. CO2 exhibited a similar a-C formation-preventing role in C2H4-supplied growth and achieved higher-purity SWCNTs with higher concentration of C2H4 than the case of H2O. However, in contrast to the other combinations, CO2 injection in the C2H2-supplied growth significantly enhanced the formation of a-C rather than the removal of it while the yield of SWCNTs was also increased, indicating the occurrence of the dehydration reaction between CO2 and C2H2. The present findings will lead to efficient growth of high-quality SWCNTs from nonmetallic growth seeds with the use of growth enhancers.

Atomic-Scale Evidence of Catalyst Evolution for the Structure-Controlled Growth of Single-Walled Carbon Nanotubes.

ConspectusKnowing how nanomaterials nucleate and dynamically evolve at the nanoscale is crucial to understanding and in turn controlling the structure and properties of a wide variety of materials, among which single-walled carbon nanotubes (SWCNTs) with chirality-dependent properties is a typical example. Catalyst takes a central role in guiding the SWCNT growth. An in-depth understanding of the growth mechanism of SWCNTs requires knowledge of the catalyst dynamic behavior during the chemical vapor deposition process, where real-time atomic-scale observations are needed. The high spatial, temporal, and energy resolution makes the state-of-the-art aberration-corrected environmental transmission electron microscope (ETEM) a superior tool for tracking the catalyst evolution and the SWCNT growth.Several key factors and processes, including the catalyst stability, carbon diffusion pathway, nucleation site, and growth modes of nanotubes, greatly influence the structure of SWCNTs. This Account summarizes our recent progress in the ETEM investigation of the dynamic catalyst behavior and nucleation of SWCNTs. We first compare the different growth modes of SWCNTs on two types of catalyst-stable solid intermetallic Co7W6 and unstable monometallic catalysts. Then we address the origin of different growth modes and chirality selectivity by revealing the atomic-scale stability and evolution of catalysts under carbon feed conditions and the observation of the in situ growth of SWCNTs on catalysts. We also discuss the catalyst-support interaction and the possible influence on SWCNT growth. In the end, we summarize the present achievements and future challenges.We carefully compare the difference in the ordinary Co catalyst and Co7W6 catalyst which has shown great chirality selectivity in SWCNT growth. Direct imaging by ETEM demonstrated that solid catalysts initiated the growth of SWCNTs with diameters smaller (dNT) than those of the catalyst particles (dNP) (dNT < dNP), whereas molten catalyst nanoparticles produced SWCNTs with similar diameters (dNT ≈ dNP). ETEM combined with in situ synchrotron X-ray absorption spectroscopy demonstrated that the Co7W6 catalyst maintained a solid stable structure under carbon feed conditions at 700-1000 °C, demonstrating the feasibility in acting as a structure template to grow SWCNTs. By contrast, the state and composition of the Co catalyst were changing during SWCNT growth. The near-surface lattice spacings of Co7W6 remained unchanged under carbon feed condition with carbon diffusion on the surface, whereas the solid Co catalyst underwent dynamic expansion and contraction due to carbon penetration into and precipitation out of Co nanoparticles. These two different pathways of carbon diffusion on or in catalysts indicate the distinctly different growth mechanisms of SWCNTs: the epitaxial growth of SWCNTs with specified chirality on the facets of Co7W6 nanocrystals and the nonselective growth of SWCNTs by the Co catalyst with Co/CoC3 as the active species. Besides the SWCNT-catalyst interface, the catalyst-support interface is also of importance in SWCNT growth. The atomic-scale information on catalyst dynamics provides a deep mechanistic understanding of SWCNT growth and will boost the development of the structure-controlled synthesis of SWCNTs and other nanomaterials.

Multi-nucleation of single-walled carbon nanotubes in floating catalyst chemical vapor deposition

Floating-catalyst CVD (FCCVD) is a highly promising technique for the scalable synthesis of single-walled carbon nanotubes (SWCNTs). The number concentration (NC) of catalyst particles is one of the process parameters which mainly influences yield of as-produced SWCNTs. In the conventional FCCVD methods, catalyst particles are usually synthesized by thermal decomposition of organometallic compounds inside the FCCVD reactor. Being in-situ catalyst formation in conventional FCCVD, it is an open question that how many SWCNTs a single catalyst particle can grow? Herein, we employed our novel spark discharged based FCCVD technique which decouples the formation of catalyst particles and SWCNTs synthesis into two subsequent process to investigate effects of catalyst NC on SWCNTs synthesis. The NC of both catalyst particles and SWCNTs in gas-phase were quantitatively compared using differential mobility analyzer (DMA). The DMA measurements corroborated with atomic force microscopy and X-rays photoelectron spectroscopy revealed that under carefully chosen reaction conditions in FCCVD, a single catalyst particle can grow more than one SWCNTs. We propose that after the growth of 1st SWCNT, the catalyst particle detach itself to start nucleating 2nd SWCNT and this process continues as long as the catalyst remains in the growth region of the FCCVD reactor.